1. Field of the Invention
The present invention relates to an improved method and apparatus for electively plating a material and, specifically, to a method and apparatus for plating selective locations on one surface of a metal plate.
2. Description of the Prior Art
As an example, the semiconductor chip bonding portions and the wire bonding portions of a lead frame for semiconductor devices must be plated with gold or silver. Since such plating elements are costly, present practice is to plate only the necessary portions of the frame.
A method and apparatus for selectively plating a material used for such purposes are explained in the following with reference to the drawings where FIGS. 1A and 1B are views showing an internal structure of a conventional apparatus for selectively plating a material. FIG. 1A is a cross-sectional view taken along line IA--IA of FIG. 1B and FIG. 1B is a cross-sectional view taken along line IB--IB of FIG. 1A. In these Figures, outer case 2 having discharge pipe 1 in the bottom includes upper lid 4 provided with one or more through-holes 3. Sealing rubber plate 6 having common openings 5 of a shape defining the portions to be plated, mask supporting plate 7 and mask plate 8 are mounted on upper lid 4. Sealing rubber plate 6, mask supporting plate 7 and mask plate 8 are adhered together by a binder so as to be an integral member independent of outer case 2. Pressing plate 11 with rubber plate 10 adhered to the lower surface is adapted to be movable up and down by suitable means as an air cylinder. Then, material 9 to be plated is mounted on mask plate 8 with the portions to be plated positioned to accurately concide with the openings 5. Pressing plate 11 is then pressed down to bring mask plate 8 and rubber plate 10 into close contact with the material 9 to be plated.
In the bottom of inner case 12 contained within outer case 2 is plating solution feeding pipe 13 connected to a plating solution circulating pump (not shown). Located in the upper portion of inner case 12 are nozzles 14 having respective circular plating solution jetting ports in positions corresponding to through-holes 3 in upper lid 4. Inner case 12 is further provided with baffle 15 for controlling the flow of the plating solution. A material insoluble in the plating solution such as, for example, platinum or titanium coated with platinum is used for nozzles 14 so that an electric current may be passed simultaneously with the jetting of the plating solution by using the nozzles as anodes and material 9 to be plated as a cathode.
A defect of the conventional method and apparatus is that, since the anode is insoluble, the amount of metal ions of the plating element in the plating solution will be reduced as the plating is continued. Therefore a metallic salt which contains the plating element must be added to the solution from time to time. A second defect is caused by the insoluble anode the same as in the first defect and occurs as a result of anode oxidation on the nozzle surface. For example, when the plating solution is a cyanide bath, cyanide ions (CN.sup.-) will be decomposed into carbonate ions (CO.sub.3.sup.2-) and nitrogen gas (N.sub.2). As a consequence, the cyanide ion concentration will be reduced and the carbonate ion concentration will increase. However, although the reduced cyanidle ion concentration can be compensated by the addition of potassium cyanide (KCN) or sodium cyanide (NaCN), the carbonate ion concentration will continue to increase further until normal plating is significantly obstructed. Therefore it is necessary to renew the plating solution before such obstruction occurs.
A third defect is that since the shapes of the portions to be plated on material 9 vary depending on the particular use of the material, difficulties may be encountered in obtaining uniformity in the plating. For example, if the shape of the portion to be plated is circular, a plating of a sufficiently uniform thickness can be made with a circular nozzle such as nozzle 14 in the conventional plating apparatus. However, if, for example, the shape of the portion to be plated is rectangular and a circular nozzle is utilized, the amount of the plating solution fed to the vicinity of the short side of the rectangle will be insufficient and thus the plated thickness will be large in the center but thin on the short side of the plating. Therefore, it is desirable to use a nozzle corresponding in shape to the portion to be plated in order to produce a plating of uniform thickness. However, as was mentioned above, nozzle 14 in the conventional apparatus for selectively plating a material is of platinum and is therefore quite expensive. Consequently, it has been prohibitively expensive to maintain an inventory of the many types of nozzles corresponding to the types of shapes of the portions to be plated.